The present invention relates generally to touch screen controllers, and more particularly to improved pen-touch detection circuits and methods therein which avoid the amount of time and the amount of power consumption and communication burden on a host processor which are associated with false pen-touch occurrences.
The prior art is believed to include commonly assigned U.S. Pat. No. 6,246,394 entitled “Touch Screen Measurement Circuit and Method” issued Jun. 12, 2001 to Kalthoff et al., incorporated herein by reference. Also, commonly assigned U.S. Pat. No. 6,738,048 entitled “Touch Screen Controller” issued May 18, 2004 to Bernd M. Rundel, also incorporated herein by reference, is indicative of the state of the art.
As shown in “Prior Art” FIG. 1 herein, the '394 patent discloses a touch screen digitizing system which includes a touch screen unit or assembly 30,31 including a first resistive screen 30 with opposed x+ and x− terminals, a second resistive screen 31 with opposed y+ and y− terminals, and an ADC 22. The various terminals of touch screen assembly 30,31 are connected to corresponding terminals of a touch screen controller (TSC) chip 1A including a first switch 19 which is coupled between a first reference voltage (ground) and the x− terminal, and a second switch 18 which is coupled between the x+ terminal and a second reference voltage +VCC for energizing the first resistive screen 30. A third switch 21 is coupled between ground and the y− terminal, and a fourth switch 20 is coupled between the y+ terminal and VCC for energizing the second resistive screen 31. Switching circuitry 15,17 couples an input of the ADC 22 to the y+ terminal while the first resistive screen 30 is energized and the second resistive screen 31 is not energized, and also couples the input to the x+ terminal while the second resistive screen 31 is energized and the first resistive screen 30 is not energized. More specifically, the various terminals of the resistive screens 30 and 31 are connected to the drains of the various corresponding driver transistors 18, 19, 20 and 21. The structure provides continuous calibration of the full-scale analog touch screen output of the full-scale digital output of the ADC 22 by means of conductors 42, and also includes a data register 48 which receives/updates the analog to digital conversion results from ADC 22 so they are available to be read by the host processor by a control/data bus 40. Control circuit 41 also can generate a processor interrupt request signal IRQ on conductor 47.
FIG. 2 shows a simplified diagram prior art pen-touch detection circuit 2A which is included in touch screen controller 1A of FIG. 1. Touch screen assembly 30,31 in FIG. 2 includes the x resistive sheet 30 and the y resistive sheet 31 as shown in FIG. 1. Pen-touch detection circuit 2A includes N-channel driver transistor 21 of FIG. 1 coupled between y-conductor 25 and ground, with its gate connected by conductor 46 to control unit 41.
x+ conductor 27 is coupled to the source of N-channel transistor 32, the drain of which is connected by conductor 36 to one terminal of a pull-up resistor 39, to the input of a buffer circuit 38, and to the drain of a N-channel transistor 35 having its source connected to ground. (As a practical matter, N-channel transistor 32 could be part of a CMOS transmission gate which includes a P-channel transistor connected in parallel with N-channel transistor 32.) The other terminal of pull-up resistor 39 is connected to VDD. The gate of transistor 32 is connected to the output of an inverter 33 having its input connected by conductor 34 to control unit 41. The gate of transistor 35 is connected by conductor 45 to control unit 41. Transistor 35 is turned on after a low level of PENTOUCH on conductor 37 is achieved as a result of detection of a touch point Q in order to hold that level after transistor 32 is turned off to allow the various driver transistors to be controlled so as to allow analog to digital conversion of the various touch screen terminal voltages. The output of buffer circuit 38 produces a pen-touch signal PENTOUCH on conductor 37 and applies it to an input of control unit 41. Control unit 41 produces the interrupt request signal IRQ on conductor 47 and applies it to an interrupt request input of host processor 3.
Prior art touch detection circuit 2A in FIG. 2 indicates whether or not there presently is a touch point Q at which there is electrical contact between the x resistive sheet 30 and the y resistive sheet 31 (FIG. 1) due to pen-touch pressure at on touch screen assembly 30,31 at the touch point Q. When transistor 21 is turned on, pen touch detection circuit 2A is ready for pen-touch detection operation. There is a current path from VDD to ground, through pull-up resistor 39, through transistor 32, x+ port conductor 27, the x and y touch screen resistances connected by the electrical contact caused by touch point Q, and driver transistor 21.
The pen-touch detection operation of touch screen controller 1A of FIG. 1, which includes pen-touch detection circuit 2A in FIG. 2, is generally indicated in blocks 50 and 51 of FIG. 3. If the determination of decision block 51 in FIG. 3 is negative, the algorithm performed by pen-touch detection circuit 2A returns to the beginning of the pen-touch detection operation indicated in block 50 and “idles” until an affirmative determination is made in decision block 51. When such an affirmative determination is made in accordance with decision block 51, touch screen controller 2A generates an interrupt request signal IRQ and sends it via conductor 47 to host processor 3, as indicated in block 52.
The program then goes to decision block 53 and simply waits for host processor 3 to decide to send a convert command in response to the interrupt request of block 52. When the convert command eventually is received, touch screen controller 1A proceeds with analog to digital conversion of the present x and y touch point coordinate voltages, irrespective of whether or not they are still valid. After the analog to digital conversion is complete, the operation of touch screen controller 1A and pen-touch detection circuit 2A returns to the beginning of pen-touch detection block 50 as indicated in FIG. 3.
From the beginning of an interrupt request sent by touch screen controller 1A on conductor 47 until the time it receives a “convert” command (i.e., a command from host processor 3 to cause touch screen controller 1A to begin analog to digital conversion of the touch point coordinate voltages on conductors 24, 25, 26 and 27), touch screen controller 1A is in a “wait mode” that may last so long that the pen-touch point Q has disappeared due to loss of adequate pen-touch pressure on the surface of touch screen panel assembly 30,31. This means that the interrupt request IRQ caused by detection of the now invalid pen-touch is also invalid. In the case of touch screen controller 1A being set to a wait mode as indicated in decision block 53, touch screen controller 1A must wait until host processor 3 decides to send a convert command back to touch screen controller 1A and causes it to perform an analog to digital conversion of the invalid x and y terminal voltages before touch screen controller 1A is able to again detect any further pen touch pressure points Q. Typically, there are numerous such “false touches” on touch screen panel 30,31 which causes the occurrence of numerous wait mode states of touch screen controller 1A. This causes numerous time-consuming and power-consuming analog to digital conversions of the invalid pen-touch points, thereby causing inefficient touch screen controller operation.
Thus, conventional touch screen controller 1A (which includes pen-touch detection circuit 2A) simply “waits” for host processor 3 to send it a “convert” command in response to the interrupt request, to cause touch screen controller 1A to perform the needed analog-to-digital conversion(s) of the coordinates of the present touch point. But there is no continued pen-touch detection operation by pen-touch detector circuit 2A during the “wait” times. The low level of the signal PENTOUCH on conductor 37 is latched into logic circuitry of control unit 41, and host processor 3 eventually responds to the interrupt request IRQ by sending a convert command to touch screen controller 1A, which then converts the analog x and y touch point coordinate voltages into corresponding digital touch point coordinates which are then stored in data register 48 (FIG. 1) and read by host processor 3 via data bus 40.
If the touch point Q disappears, the signal PENTOUCH goes to a high logic level, which causes control unit 41 to turn off transistor 32 and turn on transistor 35 to hold the pen touch signal level PENTOUCH while transistor 32 is off. In response to the eventual convert command from host processor 3, the appropriate driver transistors are turned on as mentioned above with reference to FIG. 1, but since no meaningful analog signals representing x and y coordinates of a valid touch point Q are presently being generated, the analog to digital conversion results are meaningless.
Host processor 3 then is required to read the meaningless conversion results due to the false pen-touch detection before going back to the pen-touch detection operation according to blocks 50 and 51 in FIG. 3 to determine that the previously detected touch point is now invalid. Furthermore, host processor 3 must also perform the somewhat complicated steps of recognizing and disregarding the invalid conversion results that it has read from data register 48 in touch screen controller 1A.
Another problem with the foregoing prior art pen-touch circuit and procedure is that if the present touch point Q disappears, i.e., there is a false touch point detection, and then a substantial amount of time and power are wasted performing a meaningless analog to digital conversion of the false touch point.
Thus, there is an unmet need for an improved pen-touch detection circuit and method and a touch screen controller which avoids the need for a host processor to read meaningless analog to digital conversion results that are caused by false pen-touch detection.
There also is an unmet need for an improved pen-touch detection circuit and method that avoids the need for a host processor to perform the steps of recognizing and disregarding invalid analog to digital conversion results read from a touch screen controller.
There also is an unmet need for an improved touch screen controller and pen-touch detection circuit and method that avoid wasting of a substantial amount of time and power incurred in performing meaningless analog to digital conversions of false or invalid touch points and incurred in associated bus communications with a host processor.